Etiology of neuroanatomical correlates of reading disability

Mental rotation of letters, pictures, and three-dimensional objects in German dyslexic children

This study examines mental rotation ability in children with developmental dyslexia. Prior investigations have yielded equivocal results that might be due to differences in stimulus material and testing formats employed. Whereas some investigators found dyslexic readers to be impaired in mental rotation, others did not report any performance differences or even superior spatial performance for dyslexia. Here, we report a comparison of mental rotation for letters, three-dimensional figures sensu Shepard and Metzler, and colored pictures of animals or humans in second-grade German dyslexic readers. Findings indicate that dyslexic readers are impaired in mental rotation for all three kinds of stimuli. Effects of general intelligence were controlled. Furthermore, dyslexic children were deficient in other spatial abilities like identifying letters or forms among distracters. These results are discussed with respect to the hypotheses of a developmental dysfunction of the parietal cortex or a subtle anomaly in cerebellar function in dyslexic readers.

Neurobiologische Grundlagen der Lese-Rechtschreib-Schwäche

Zusammenfassung: Diese Überblicksarbeit stellt die wichtigsten neurobiologischen Theorien zu Ursachen der entwicklungsbedingten Lese-Rechtschreib-Schwäche und daraus abgeleitete Therapieansätze vor. Es wird zunächst ein kurzer Überblick über die Symptomatik und die vermuteten genetischen Grundlagen der Störung gegeben. Die phonologische Defizithypothese als eine allgemein akzeptierte, auf der kognitiven Ebene angesiedelte Theorie wird vorgestellt. Neurobiologische Modelle versuchen zu erklären, auf welchen neuroanatomischen/physiologischen Ursachen das phonologische Defizit beruht. Die magnozelluläre Defizithypothese, die Hypothese eines grundlegenden Verarbeitungsdefizits für sich zeitlich schnell verändernde Reize, die cerebelläre Defizithypothese sowie neuroanatomische Unterschiede zwischen normalen und legasthenen Lesern werden vorgestellt. Neurobiologisch fundierte Therapieansätze der Lese-Rechtschreib-Schwäche versuchen, in Ergänzung zu traditionellen, eher linguistisch orientierten Therapien die betroffenen neuronalen Systeme in ihrer Funktionsfähigkeit zu trainieren und dadurch die Voraussetzungen für die betroffenen Kinder zu schaffen, vom normalen Lese- und Schreibunterricht zu profitieren. Einige dieser Ansätze werden dargestellt. Abschließend wird darauf hingewiesen, dass eine differenzielle Zuweisung von Patienten zu bestimmten Therapieprogrammen auf der Basis einer fundierten (Ursachen-)Diagnostik in der Praxis stärker als bislang üblich durchgeführt werden sollte.

Recognition memory for unfamiliar faces does not differ for adult normal and dyslexic readers: an event-related brain potential study

OBJECTIVE

Differences in recognition memory for unfamiliar faces between adult developmental dyslexic (n=12) and normal readers (n=12) were studied by means of event-related brain potentials.

METHODS

Subjects performed a continuous face recognition task, in which 240 unfamiliar faces were presented on a computer screen, 100 of which were repetitions. For each face, subjects had to indicate whether it was presented before or not. Performance did not differ between normal and dyslexic readers. Old/repeated faces elicited more positive event-related potentials (ERPs) starting 250 ms poststimulus. These were analyzed in two time-windows encompassing the early (250-450 ms) and the late phase (450-750 ms) of the old/new effect.

RESULTS

No group difference in amplitude or topography of the old/new effect emerged. However, ERPs for all faces were more positive for normal compared with those of dyslexic readers.

CONCLUSIONS

These results show that a previously described recognition memory deficit for words in dyslexic readers is likely to be specific for verbal material.

Memory performance of children with dyslexia: a comparative analysis of theoretical perspectives

This study examined the memory performance of children with reading disabilities (RD) using methodology representative of three theoretical perspectives on RD subtypes: the phonological deficit, dual route, and phonological-core variable-difference models. Analyses compared the serial memory, verbal learning, and abstract visual-spatial memory performance of 45 children with RD to that of chronological-age (CA)- and reading-level (RL)-matched controls, using subtype identification methods from each of the theoretical models to classify children with RD. Phonological deficit and dual route comparisons indicated that children with RD, regardless of subtype, performed more poorly than CA- and better than RL-matched participants on all memory tasks. Phonological-core variable-difference methodology yielded three RD subtypes, two of which exhibited distinctive memory deficits relative to both CA and RL control groups. The phonological-core variable-difference model accounted for more variance in memory performance than either of the other two models.

Developmental dyslexics show altered allocation of attention in visual classification tasks

OBJECTIVES

Event-related brain potentials (ERPs) were recorded to investigate allocation of attention in adult developmental dyslexics.

SUBJECTS AND METHODS

Twelve adult developmental dyslexics and 12 matched normal controls performed three visual choice reaction tasks. In the passive oddball condition, subjects watched two different simple visual stimuli presented with 87.5 and 12.5% probability. In the active oddball condition, participants responded to the low-probability target stimulus. In the active 50/50-condition, both stimuli were presented with 50% probability and a response was required to the target stimulus only.

RESULTS

No group differences emerged for performance, P300 latency or laterality and for N200 amplitude, latency or laterality. An enhancement of P300 amplitude with a frontal distribution was found for NoGo (standard)-stimuli in both active conditions for the dyslexic sample.

CONCLUSION

Results are discussed in the context of deviances in allocation of attentional resources in dyslexic readers.

Heritability of lobar brain volumes in twins supports genetic models of cerebral laterality and handedness

Although the left and right human cerebral hemispheres differ both functionally and anatomically, little is known about the environmental or genetic factors that govern central nervous system asymmetry. Nevertheless, cerebral asymmetry is strongly correlated with handedness, and handedness does have a significant genetic component. To explore the relative contribution of environmental and genetic influences on cerebral asymmetry, we examined the volumes of left and right cerebral cortex in a large cohort of aging identical and fraternal twins and explored their relationship to handedness. Cerebral lobar volumes had a major genetic component, indicating that genes play a large role in changes in brain volume that occur with aging. Shared environment, which likely represents in utero events, had about twice the effect on the left hemisphere as on the right, consistent with less genetic control over the left hemisphere. To test the major genetic models of handedness and cerebral asymmetry, twin pairs were divided into those with two right handers and those with at least one left hander (nonright handers). Genetic factors contributed twice the influence to left and right cerebral hemispheric volumes in right-handed twin pairs, suggesting a large decrement in genetic control of cerebral volumes in the nonright-handed twin pairs. This loss of genetic determination of the left and right cerebral hemispheres in the nonright-handed twin pairs is consistent with models postulating a right-hand/left-hemisphere-biasing genetic influence, a "right-shift" genotype that is lost in nonright handers, resulting in decreased cerebral asymmetry.